Liu Rixin & Song Ximei & Liu Ting
(Beijing Saiwei Meigao Technology Co., Ltd.)
1、Overview
The baking temperature for powder coating significantly affects the color variation of the coating film. The curing temperature and curing time of the powder coating process are tested using a furnace temperature monitoring instrument. Regular temperature testing is important to ensure that all products achieve the required curing temperature and curing time, thereby ensuring coating quality. When product quality issues arise, it is possible to quickly identify and resolve potential problems in the coating baking process.
2. Control of temperature and baking temperature management
The control panel for the baking line typically displays the baking line's temperature and speed, and may also show the temperature at the exhaust outlet of the combustion chamber. This temperature is measured using thermocouples (or thermistors) installed on the baking line.
This control regulates the temperature at a specific fixed location on the baking line. However, the temperature of the baked product and this test point are not in the same physical space.
The thermocouples (or thermistors) require regular accuracy checks, typically on a monthly basis. They should be replaced every two years.
The baking temperature and baking time refer to the actual temperature of the workpiece and the cumulative time required to maintain that temperature, rather than the set temperature of the oven and the time the workpiece spends inside the oven. Quality control requirements ensure that all products achieve the specified curing temperature and curing time, with the maximum temperature not exceeding the maximum temperature allowed for the powder.
The temperature of the coating line must be regularly monitored using a temperature tracking instrument. Typically, at least the temperatures of the upper, middle, and lower products, as well as the flue gas temperature, should be measured. Before commissioning, after maintenance and re-use, when replacing heating equipment, or when replacing temperature sensors (thermocouples/thermistor), temperature curve testing must be performed before resuming production. Routine testing should be conducted at least once per month.
3、SMT oven temperature monitoring system
3.1 Furnace Temperature Monitoring System
Furnace temperature monitoring systems can be divided into various types, such as the simple four-channel (SMT-4) furnace temperature monitoring system and the wireless seven-channel (SMT-X7-4M-300-K) furnace temperature monitoring system. The most suitable furnace temperature monitoring system should be selected based on the number of measurement points, process temperature, and time requirements.[1].
3.2 Testing Principle
The furnace temperature monitoring system is equipped with an insulating box for thermal insulation, and the product is placed inside the box and enters the furnace. Temperature data is recorded within the furnace temperature monitoring system. The test temperature is typically measured using K-type thermocouples. Various thermocouple attachment methods are available, including adhesive thermocouples, clamp-on thermocouples, and magnet-mounted thermocouples, depending on the process requirements. The test temperature sensors can measure the temperature of the furnace atmosphere at various points and the surface temperature of the product at multiple locations. All temperature data within the entire furnace is recorded in the furnace temperature monitoring system. After the product is removed from the furnace, communication with a computer is established to read all the data from the process. A dedicated software is used to analyze the test results, determining the curing time and the maximum temperature during the curing process for a specified value (determined by customer requirements), where the curing temperature is higher than a certain value.
The wireless oven temperature monitoring system utilizes both WiFi and USB communication, allowing real-time temperature data to be integrated into the control system.
4、Testing methods and procedures
4.1 Establish Sampling Period
When configuring the sampling period on the software or furnace temperature tracking instrument display, options include 1s, 2s, 5s, 10s, 20s, etc. It is typically set to 1s.
4.2 Securing thermocouples on the workpiece
Secure the thermocouple's hot end to the surface of the test specimen, and measure the gas temperature using a gas-fired thermocouple test furnace.
Adhere thermocouples: Use high-temperature tape to attach the thermocouple to the workpiece surface. This method is particularly suitable for metal, plastic, and other shaped workpieces with thicknesses of 2mm or less. A key feature is its short response time.
Clamp-type thermocouples: Easy to fix, suitable for various aluminum alloy components.
- Suitable for ferrous metal workpieces only; not suitable for stainless steel, aluminum, plastic, etc.- Disadvantage: long response time.
At a minimum, test the internal furnace atmosphere temperature.
4.3 Furnace Entry Test
Before entering the furnace, turn on the furnace temperature monitoring device and start recording. The furnace temperature monitoring device and the workpiece are placed in the furnace together, recording and storing the temperature variations of the powder coating applied to the workpiece throughout the curing process. After removing the workpiece from the furnace, stop recording and then remove the thermocouple.
4.4 Download and analyze the data
After the temperature tracking instrument has finished its cycle, the recorded temperature data is downloaded to the computer via a data cable, and then analyzed using specialized analysis software.
4.5 Storage Test Temperature Data
Simultaneously record the actual oven temperature and the conveyor line speed. Analyze the test results based on this data.
5. Actual Test Examples
5.1 Aluminum Alloy Wheel Hub Coating Temperature Test
Powder coatings are increasingly used for aluminum alloy wheel rims. Currently, the main aluminum wheel rim industry in China uses transparent powder for surface coating. Coating is the final process in aluminum alloy wheel rim production, and its primary function is to enhance the corrosion resistance and aesthetic appearance of the rims in various environments during driving. Different colors can be applied according to user requirements. After each layer of coating, including primer, color coat, and transparent layer, is sprayed, it needs to be cured. Therefore, each set of coating equipment is connected to a curing oven and a corresponding cooling oven. The actual temperature curve inside the oven can be tested and adjusted using an SMT oven temperature tracking instrument to ensure that the temperature curve meets the process requirements.
Figure 1 shows the curing oven temperature curve for the hub coating. The maximum air temperature reaches 231.7°C, which is slightly higher than the maximum temperature allowed for the powder coating. Due to the thickness of the workpiece, if the curing time at high temperatures cannot meet the requirements, the speed of the production line can be reduced.
Figure 1: Aluminum alloy wheel hub coating temperature curve
5.2 Appliance Coating Temperature Test
The powder curing process used for appliance painting is 180±10℃, with a baking time of 15±5 minutes, which is a normal curing process.
However, these two processes are interconnected. During the initial setup of the equipment, a SMT oven temperature tracking instrument is used to measure the maximum surface temperatures of the top, middle, and bottom three points of the workpiece, as well as the cumulative time, and to adjust the curing oven set temperature and conveying chain speed (which determines the workpiece's travel time within the oven) until the desired curing process requirements are met. This allows for the establishment of a corresponding relationship between the two. Figure 2 shows the coating temperature curve for the refrigerator outer shell. This temperature curve is reasonable.
Figure 2: Appliance painting temperature curve
5.3 Color-Coated Panel Coating Temperature Test
Color-coated panels are products formed by applying organic coatings on a substrate surface through a continuous production process, followed by baking. As a composite material, color-coated panels possess high strength, ease of forming, and excellent corrosion resistance. Furthermore, organic coatings can be formulated into various colors and produced with patterns such as wood grain and leather grain using techniques like printing and embossing, thus providing excellent decorative properties. Consequently, color-coated panels are increasingly used in construction, home appliances, and transportation.
In the production of coated panels, curing is a critical process. After applying the coating to the substrate, heat is applied to allow the solvents in the wet coating to evaporate, while simultaneously causing the organic materials to undergo polymerization or crosslinking reactions under the influence of a catalyst. Curing not only ensures that the coating is firmly bonded to the substrate and exhibits certain mechanical and physical properties, but also directly influences the product performance of the coated panel. Figure 3 shows the temperature curve for aluminum panel coated panels.
Figure 3: Aluminum plate roll coating temperature curve
5.4 Aluminum Profile Coating Temperature Testing
The aluminum profile coating line mainly consists of two types: vertical and horizontal. For the horizontal type, it is recommended to use a 4-channel furnace temperature tracking system, and for the vertical type, a 7-channel system is recommended. During curing, the temperature and time must be strictly followed according to the specified parameters. If the temperature is too low or the time is too short, the powder will not form a pattern, and the mechanical properties will also significantly decrease due to incomplete curing. The powder curing process is 180±10℃, with a baking time of 15±3 minutes, which is the normal curing process.
Figure 4 shows the actual test curve of a horizontal aluminum profile spray painting line.
Figure 4: Coating temperature curve for aluminum profiles
The maximum temperatures on the upper, middle, and lower aluminum surfaces are 251.0°C, 242.9°C, and 242.9°C, respectively. The baking time for temperatures above 220°C is 13 minutes, 12 minutes, and 12 minutes, respectively.
The furnace gas temperature has a localized high temperature, reaching 291.3℃. This temperature does not affect the product quality during normal operation of the production line. However, if the production line stops moving and the aluminum profiles in this high-temperature area are baked, the quality of the baked profiles will be affected. It is necessary to eliminate this localized high-temperature area by adjusting the heat distribution of the baking line or reducing the temperature of the high-temperature burner outlet.
6. Conclusion
This article introduces some application examples of temperature management and SMT oven temperature monitoring in baking lines. It is recommended to regularly inspect the control thermocouples (or thermistors) in the coating line, and to replace them every two years. The temperature curve of the coating line should be tested once a month. After a major overhaul of the baking line, before restarting production after shutting down the oven, it is necessary to use the oven temperature monitoring instrument to first check whether the oven temperature curve is compliant before starting production.
References
[1] Song Ximei, Liu Rixin, Application example and calibration of SMT oven temperature tracking instrument for coatings and coatings, Modern Coatings, Vol18, 3, 49-51, 54